The present invention relates in general to a printer and more particularly to an apparatus and method for controlling the movement of a printhead assembly.
Conventional dot matrix printers have become very popular because they provide high quality characters and bit images at a relatively low cost. Dot matrix printers of a serial type operate by driving a head fixing base or carriage having a printhead thereon in a printing direction. The movement of the carriage is controlled by print timing signals, which are generated according to how fast or far the carriage moves relative to a slit plate.
In conventional printers, each line is printed only while the carriage is moving at a constant speed in a printing area (FIG. 17). A slit detector generates encoder signals as the carriage moves relative to the slit plate, which has slits a predetermined distance apart. The generated encoder signals are used as starting points and corresponding print timing data is set in a timer circuit in accordance with print instruction data received from an external device at timings corresponding to the respective printing modes of the printer. Once the timer circuit is set, print timing signals are generated to control the further movement of the carriage and the printhead striking the paper.
As shown in FIG. 18 herein, Japanese Patent Publication (Tokkai) No. 3-2059 of Heisei discloses a print timing control method for correcting variances in printing positions between respective dots in a transverse direction. This reference employs delay circuits 301 for controlling a drive circuit 302 which drives the respective head pins of a printhead 303. The delay circuits 301 change the delay time according to the printing direction to compensate for variances in the printing position.
Further, head pins which are provided in the printhead of a serial type printer are respectively driven at a predetermined cycle. The time for energizing a head pin driving solenoid or for applying voltage to a head pin driving piezoelectric element during one cycle is fixed independent of the moving speed of the carriage.
There is a demand for increased print speed which has resulted in an increase in the printing speed during the constant speed movement of the carriage. The demand for increased print speed has also resulted in printing during the accelerating and decelerating movements of the carriage (i.e., in the periods shown by oblique lines in FIG. 17). However, to print during the accelerating and decelerating movements of the carriage, print timings must be generated relative to the moving speed of the carriage which changes momentarily. Furthermore, during the accelerating and decelerating periods, the movement of the carriage is greatly influenced by the variations of loads occurring while controlling the print carriage drive mechanism. In conventional printers, the movement of the carriage during the accelerating and decelerating periods is based on timing signals corresponding to the print area. As a result, the printing quality is not satisfactory.
In addition, in the conventional method, the printing operations are performed in such a manner that the carriage moves without stopping to strike the head pins against the paper via a ribbon. Therefore, as the printing speed of the printer increases, the contact distance on the paper between the head pin and the paper increases thereby spreading the shapes of dots printed on the paper. The result is that the printing quality is deteriorated. Further, the change in the shape of the dot is clearly recognized as a change in the shape of a character or image especially when the printing is performed in the accelerating or decelerating movements of the carriage. The change in the dot shape is undesirable from the viewpoint of the printing quality.